1. Field of the Invention
The invention relates to micrometers and, more particularly, to an electro-optical micrometer capable of extreme accuracy.
2. Description of the Prior Art
Typical mechanical measuring devices such as micrometers, dial indicators and vernier calipers are suitable for making measurements that are accurate within thousandths of an inch. These devices are inexpensive, lightweight, compact, and relatively rugged. Accordingly, they are useful for many types of applications. Unfortunately, when greater accuracy of measurement is desired, these mechanical devices are inadequate.
In an attempt to increase the accuracy of thickness measurements, various optical and electro-optical micrometers have been developed. The patent to Bentley et al., U.S. Pat. No. 2,549,566, is an early example of an optical micrometer wherein a beam of light is reflected from a mirror and is directed through a transparent, semi-transparent, or translucent workpiece such as a plate of glass, plastics, or other, similar materials. An image of the workpiece is viewed through an eyepiece, which image is brought into focus by means of the adjustment of a micrometer. Due to the amplification permitted by the beam of light and the eyepiece, the accuracy of measurement of the micrometer is increased.
Unfortunately, the device is somewhat difficult to use in that it requires judgment on the part of the operator as to when the image is in proper focus. The focusing of the beam of light and the interpretation of the results are conducted in an entirely manual manner.
A similar device is disclosed in the patent to Kettler, U.S. Pat. No. 2,773,311, wherein a vertically movable rod compresses a workpiece against the upper surface of an anvil. The upper end of the rod carries a reticle. A lamp sends a beam of light through lenses, the reticle, and thereafter through an enlarging lens system. The light discharged by the enlarging lens system is directed to angled mirrors which in turn direct the beam of light onto a viewing screen. The thickness of the workpiece can be determined visually by observing where the beam of light falls on the viewing screen. Kettler claims that the markings on the reticle can be enlarged by a factor of 1,000 due to the particular magnification of the lens system and the positional arrangement of the mirrors relative to the viewing screen.
While such a device is believed to be an improvement over that disclosed by Bentley et al., in part because non-transparent objects can be measured and because less judgment is required in making a thickness measurement, the accuracy of the device nevertheless is not as great as desired.
Various other electro-optical measuring devices are known in which a light source directs a beam of light either through a lens system or against a mirror, or both. The characteristics or movement of the beam of light are analyzed in some manner to determine the thickness of an object or the relative position of an object. The patents to Root, U.S. Pat. Nos. 2,604,004, and Bendt, 2,820,298, compare the Newtonian fringes present in a light interference pattern to determine the thickness of an object. The patent to Rieder et al., U.S. Pat. No. 4,459,749, employs phototransisters that receive a reflected light signal and thereafter send signals to amplifying and evaluating circuitry as well as a microprocessor. The microprocessor provides a digital readout of the thickness of an object being measured. The patent to Schmitt, U.S. Pat. No. 4,678,948, is similar to the Rieder et al. device in that a light source sends optical signals to photosensors which in turn send amplified signals to AND gates and thereafter to a phase discriminator. By sending the light beam through a graduated scale and a graduated scanning plate (which are attached to two different objects), the relative position of the objects can be determined.
Despite the capabilities of the referenced devices, there remains a need for a micrometer capable of extreme accuracy. Desirably, any such micrometer would be relatively inexpensive, would require little or no judgment on the part of the operator in order to be utilized properly, and would enable objects of a wide variety of sizes and shapes to have their thicknesses determined quickly and accurately.